Banners and posters advocating Hong Kong independence surfaced at more universities across the city on Wednesday, as seven student unions joined forces to condemn the removal of such material by campus authorities as a “serious erosion” of academic freedom.

The unions from City University, Hang Seng Management College, Lingnan University, Polytechnic University, Education University, Open University and Chinese University issued a joint open letter defending the proliferation of material promoting Hong Kong’s separation from China.

“Freedom of speech and thought is a God-given human right. People may disagree with views on independence, but [anyone] should enjoy the right to talk about it,” the letter said.

“The suppression of such ideas by the school not only deprives students of political rights, but is a humiliation to academic freedom.”

The controversy began on Monday with at least three large black banners bearing the words “Hong Kong independence” in Chinese and English appearing at Chinese University in Sha Tin as the new academic year kicked off.

It then spread to the University of Hong Kong, City University, Polytechnic University, Education University and Shue Yan University from Tuesday afternoon. Some were put up by student unions, while others were done anonymously.

On Wednesday, Chinese University’s student union warned it might take legal action to stop banners being removed from the campus, saying it was considering seeking a judicial review over the management’s handling of the matter. The union set a 7pm deadline on Wednesday for the university to respond, which passed without incident.

Students at Education University admitted they had hung a large black banner bearing the phrase “Hong Kong independence” outside the library of their Tai Po campus on Tuesday evening.

Union president Lala Lai Hiu-ching said the group’s primary aim was to support their counterparts at Chinese University.

Asked if the union championed independence, she said it was “an option that could not be ignored”.

“We support the discussion of different ways forward for Hong Kong,” Lai said. “But we have now demonstrated that the school will engage in political screening [of publicity materials], hurting our autonomy and freedom of expression.”

Her views were shared by Apostle Lau Chak-fung, who heads the student union of the privately funded Shue Yan University.

A “democracy wall” at its Braemar Hill campus was filled with independence posters as of Wednesday afternoon.

“We put them up after a consensus within the union that we should rally behind our peers at Chinese University,” Lau said.

If people disagreed, he added, they should respond by putting up posters of their own, instead of tearing down materials they disliked.

Lau also revealed that a meeting between the student union leaders was being arranged within the next few days to coordinate their actions.

A similar, smaller-scale stunt was carried out at City University by a student whose identity was not confirmed.

City University’s student union claimed posters advocating independence had been pasted on the Kowloon Tong campus’ “democracy wall” – a notice board for students to display materials freely – and removed by security personnel.

“The move not only destroyed the trust between the school and the union, but also the freedoms of City University students.”

A similar banner remained intact at Chinese University on Wednesday afternoon, more than a day after it had been put up for the second time.

The student union did not claim responsibility for the stunt, but vowed to intervene if school management tried to take it down.

A Chinese University spokeswoman said the management “does not agree with” Hong Kong independence, “as the Basic Law states Hong Kong is an inalienable part of China”.

The union also issued a 7pm deadline on Wednesday for outgoing vice-chancellor Professor Joseph Sung Jao-yiu and his successor in waiting, Rocky Tuan Sung-chi, to state their positions on the matter, threatening “escalating action” if they failed to respond.